Heterogeneity of Hydrologic Response in Four Mountainous Watersheds in Northwestern Rwanda

Abstract

Four mountainous watersheds within the rapidly developing Ruhengeri Prefecture in northwestern Rwanda show striking heterogeneity in their hydrologic regimes. The fertile steeplands of Ruhengeri Prefecture support some of the highest population densities in the world. They also contain the protected Parc National des Volcans, home to the endangered mountain gorilla. This study presents the first sedimentation data available for the region. Suspended sediment yield ranged widely from 124 t/km2 to 19,800 t/km2 during a six-month period from December 1987 to May 1988. The latter figure includes sediment transported during a catastrophic flood in May 1988, and is comparable to the highest measured values worldwide. Precipitation is relatively uniform throughout the region, yet runoff varied from only 19 mm to as much as 1,055 mm during the six-month comparison period. Flood events were highly localized and, in some cases, severe. Land use, geology, and topography are the primary factors in this variance. Considera- tion of catchment-specific hydrologic and sedimentation data will be important to both sustained agricultural yields and the design of appropriate floodplain infrastructures. Certain catchments characterized by alarmingly high sedimentation rates require immediate stabilization and rehabilitation.